The present invention relates to improvements in spray guns, and in particular to an airless spray gun for spraying two component chemically reactive resin forming materials, especially those which are foam forming.
One exemplary type of spray gun for plural component materials, especially those that are foam forming, is disclosed in U.S. Pat. No. 3,366,337 to Brooks et al, assigned to the assignee of the present invention. In that spray gun, means are provided for intimately mixing and spraying a pair of coreactive chemical fluids immediately prior to the situs of application, by impinging jets or streams of the reactants against each other under pressure from opposed directions into the rearward end of a relatively large mixing chamber, whereby the reactants are mixed within the chamber and discharged therefrom through an outlet orifice at a forward end thereof solely under the influence of the pressure. Mixing and spraying is accomplished without the aid of or necessity for a mechanical agitating or mixing means which might hamper or impede the flow or the resultant reaction mixture, and as a result a better foamed product is produced.
At the end of a spraying operation, mixed materials within the chamber must be quickly removed before completion of the reaction and formation of a blockage of foam in the gun. To that end, in the spray gun of said U.S. Pat. No. 3,366,337, means are provided for separately introducing a stream of solvent into the rearward end of the mixing chamber after the gun has been operated, so that the chamber and outlet orifice can be cleaned of residual material to enable further spraying operations. Disadvantages of the cleaning technique are that a separate container must be provided to collect the discharge during cleaning, it occasionally happens that the solvent contacts and mars a finished foamed product, the use of solvent adds cost to the foaming operation, and in view of environmental considerations it is not desirable that solvent be sprayed into the air. In addition, cleaning of the chamber with solvent is often less than thorough, with the result that the gun must be disassembled to remove hardened material, and customary practice contemplates soaking the gun in solvent to remove mixed reactants whenever the gun is not going to be used for a period of time.